A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field.
However, the dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques at visible wavelengths. Small features include for example those formed by multiple patterning processes, and/or pitch-multiplication. While scanning electron microscopy (SEM) is able to resolve these modern product structures directly, SEM is much more time consuming than optical measurements.
Similar challenges occur when imaging biological structures, such as cells at micron or sub-micron resolutions. The ability to image biological structures is essential for studying cellular structure and function. Imaging plays a central role in medical diagnostics, being the gold standard in pathology for the identification of many diseases from biopsy samples.
Optical microscopy is a standard method for life science imaging, as the achievable resolution allows a detailed view of intracellular structures, while the field of view can be sufficiently large to image larger structures such as cellular networks and tissue specimen. A particular advantage of visible/near-infrared light microscopy is the low interaction of the light with cells (i.e. low absorption), so that biological material can be imaged without being influenced significantly by the light itself, while the light can be detected with high efficiency so that low intensity can be used.
Since cells are very transparent to light, image contrast in pathology is typically provided by staining cells with substances that absorb specific parts of the optical spectrum, leading to colored absorption contrast.
The inventor has considered whether the techniques of coherent diffraction imaging (CDI), combined with radiation of wavelength comparable with the product structures of interest, might be applied to defect detection on modern device structures. CDI is also known as lensless imaging, because there is no need for physical lenses or mirrors to focus an image of an object. The desired image is calculated synthetically from a captured light field. Various techniques for CDI are described in the PhD thesis describing lensless imaging at EUV wavelengths is “High-Resolution Extreme Ultraviolet Microscopy” by M. W. Zürch, Springer Theses, DOI 10.1007/978-3-319-12388-2_1. A particular type of CDI is ptychography, described for example in published patent application US 2010241396 and U.S. Pat. Nos. 7,792,246, 8,908,910, 8,917,393, 8,942,449, 9,029,745 of the company Phase Focus Limited and the University of Sheffield. D. Claus et al provide an introduction to ptychography in a paper “Ptychography: a novel phase retrieval technique, advantages and its application” Proc. SPIE 8001, International Conference on Applications of Optics and Photonics, 800109 (Jul. 26, 2011); doi:10.1117/12.893512. In ptychography, phase information is retrieved from a plurality of captured images with an illumination field that is moved slightly between successive captures. Overlap between the illumination fields allows reconstruction of phase information and 3-D images. Other types of CDI can be considered also.
Successful use of ptychography requires a number of requirements to be met. Firstly, the size of the radiation spot used to illuminate the target must be controlled very precisely in order to only illuminate part of a target. This is typically done by using optical elements with a high Numerical Aperture (NA). However, this can be both challenging and costly, in particular for radiation at extreme ultra-violet (EUV) wavelengths. Alternatively, it is possible to use a small aperture. This, however, leads to a large loss in radiation flux, which is undesirable. In particular, as described above, biological material is imaged at low radiation intensities to avoid the material being influenced by the radiation. A large loss in radiation flux may render such methods unfeasible for biological imaging purposes.
A second requirement is that the transverse displacement of the radiation beam relative to the target needs to be accurately known and controlled. This requires the relative position of the optical system and the target to be precisely controlled. This increases the complexity and cost of the system. It may further increase the physical space requirements of the system, which is undesirable.
As an alternative to the use of a finite radiation spot, structured illumination patterns have been proposed. However, in particular for radiation with EUV wavelengths, this requires the production of multiple coherent radiation patterns with small feature sizes. In turn, this requires the use of optical components which must be carefully positioned and maintained.